There are many suggestions for displaying relevant data to the driver of a motor vehicle by means of a head-up display (HUD). Measured data and warning signals of importance for the driver are reflected onto a portion of the windshield which lies within the field of vision of the driver. This enables the driver to recognize data, such as speed, safe brake operation or traffic information, even if his attention is directed to the traffic situation and without having to remove his eyes from the road. The need for distance adjustment by the eyes of the driver is eliminated to a large extent.
Head-up displays require a partially transparent mirror for superimposition of the virtual image generated by the data elements on the image of the environment. A reflection on the windshield or on an additional plate (combiner) placed inside of the windshield can be used for this in a motor vehicle. In both cases reflections appear at the planes of separation between the windshield or the combiner plate and the air, which result in undesirable double images and in this way considerably impair the legibility of the data. The entry of light into the optical space of the HUD results in reflections which are also blended into the field of vision of the driver. A bright, fog-like background surface appears for the driver, which hampers the view of the traffic situation. To suppress these effects, coating of the inner boundary surface with the dielectric layer is generally suggested, by means of which one of the reflected images is enhanced and the other attenuated.
It is furthermore known to design the reflecting layers in such a way that they only reflect light of the wavelength given off by the indicator elements disposed within the dashboard. The effect of ambient light can be reduced thereby.
It is furthermore known from German Patent DE-PS 33 47 271 to polarize the luminous flux and to deflect it in the direction of the driver from a multiple coating with different refractive indices applied to the windshield, taking the Brewster angle into consideration. The application of dielectric coatings by vacuum evaporation or sputtering in high vacuum, in particular on the windshield, is very expensive. It is known that these coatings are also very sensitive to scratches and dirt, so that the legibility of the displays is impaired.